Classifications

• • G—PHYSICS
  • G10—MUSICAL INSTRUMENTS; ACOUSTICS
  • G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
  • G10K15/00—Acoustics not otherwise provided for
  • G10K15/02—Synthesis of acoustic waves
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
  • H04R1/00—Details of transducers, loudspeakers or microphones
  • H04R1/20—Arrangements for obtaining desired frequency or directional characteristics
  • H04R1/22—Arrangements for obtaining desired frequency or directional characteristics for obtaining desired frequency characteristic only 
  • H04R1/26—Spatial arrangements of separate transducers responsive to two or more frequency ranges
• • G—PHYSICS
  • G10—MUSICAL INSTRUMENTS; ACOUSTICS
  • G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
  • G10K11/00—Methods or devices for transmitting, conducting or directing sound in general; Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
  • G10K11/18—Methods or devices for transmitting, conducting or directing sound
  • G10K11/26—Sound-focusing or directing, e.g. scanning
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B5/00—Near-field transmission systems, e.g. inductive or capacitive transmission systems
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
  • H04R1/00—Details of transducers, loudspeakers or microphones
  • H04R1/20—Arrangements for obtaining desired frequency or directional characteristics
  • H04R1/32—Arrangements for obtaining desired frequency or directional characteristics for obtaining desired directional characteristic only
  • H04R1/40—Arrangements for obtaining desired frequency or directional characteristics for obtaining desired directional characteristic only by combining a number of identical transducers
  • H04R1/403—Arrangements for obtaining desired frequency or directional characteristics for obtaining desired directional characteristic only by combining a number of identical transducers loud-speakers
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
  • H04R3/00—Circuits for transducers, loudspeakers or microphones
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
  • H04R3/00—Circuits for transducers, loudspeakers or microphones
  • H04R3/04—Circuits for transducers, loudspeakers or microphones for correcting frequency response
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
  • H04R2217/00—Details of magnetostrictive, piezoelectric, or electrostrictive transducers covered by H04R15/00 or H04R17/00 but not provided for in any of their subgroups
  • H04R2217/03—Parametric transducers where sound is generated or captured by the acoustic demodulation of amplitude modulated ultrasonic waves
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
  • H04R2430/00—Signal processing covered by H04R, not provided for in its groups
  • H04R2430/03—Synergistic effects of band splitting and sub-band processing
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
  • H04R3/00—Circuits for transducers, loudspeakers or microphones
  • H04R3/12—Circuits for transducers, loudspeakers or microphones for distributing signals to two or more loudspeakers

Definitions

• This invention relates to a directional sound system and a method for processing an input signal to a directional sound system.
• the parametric loudspeaker is based on a nonlinear acoustics property (known as the parametric array effect in air) that uses ultrasound signal to carry the audible sound signal in a tight beam, just like an audio spotlight.
• a loudspeaker array as described in (ii)
• the dimension of the loudspeaker array must be significantly greater than the audio wavelength in order to achieve a good directivity.
• this means that the dimension of the loudspeaker array must be more than a meter in diameter. This approach of creating a focused sound beam hence incurs a high cost since a large loudspeaker array is required.
• a parametric loudspeaker (as described in (iii)) is able to generate a highly- directional sound beam for a low-frequency sound wave whose wavelength is much larger than the loudspeaker diameter. This is because the small-sized ultrasonic emitter in the parametric loudspeaker is able to produce a highly-directional sound beam without using a vibrating cone as opposed to conventional loudspeakers.
• Fig. 1 illustrates a parametric loudspeaker according to the prior art.
• an ultrasonic carrier signal is first modulated by a modulating input signal which is in the form of an audible sound signal. Preprocessing and modulation units are used to generate the modulated signal.
• the modulated signal is then passed to an amplifier that drives the ultrasonic emitter to project the modulated signal through a transmission medium (usually air).
• a transmission medium usually air
• the modulated signal is radiated into the transmission medium, it interacts with the transmission medium and self-demodulates to generate a tight column of audible signal.
• An audible sound beam is thus generated in the transmission medium through a column of virtual audible sources as shown in Fig. 1.
• This column of virtual audible sources forms an end-fire array of audible sources (referred to as a parametric array) that add up in phase along the propagation axis.
• Equation (1 ) The Berktay far-field model is widely used to approximate the nonlinear sound propagation by the parametric loudspeaker through the transmission medium.
• This model uses an expression as shown in Equation (1 ) to predict the far field array response of the parametric loudspeaker.
• the demodulated signal (or audible difference frequency) pressure P 2 O along the axis of propagation is proportional to the second time-derivative of the square of the envelope of the modulated signal when amplitude modulation is used.
• Equation (1 ) ⁇ is the coefficient of nonlinearity
• P 0 is the primary wave pressure
• a is the radius of the ultrasonic emitter
• p 0 is the density of the transmission medium
• C 0 is the small signal sound speed
• z is the axial distance from the ultrasonic emitter
• « o is the attenuation coefficient at the source frequency
• E(t ) is the envelope of the modulated signal.
• Equation (1) the nonlinear sound propagation results in a distortion in the demodulated signal. This in turn results in a distortion in the audible signal generated by the parametric loudspeaker, hence affecting the performance of the parametric loudspeaker.
• the current parametric loudspeaker technology is severely limited by the technological constraints of ultrasonic emitters. One such technological constraint is the small usable low-frequency bandwidth of the ultrasonic emitters.
• Digital signal processing techniques have previously been proposed to overcome the technological limitations of the parametric loudspeaker technology. These techniques usually involve pre-processing algorithms which can be programmed in a digital signal processor to enhance, equalize and compensate for any distortion in the audio quality of the signal before sending the processed signal to the ultrasonic emitter. Examples of such techniques are described below.
• Fig. 2 shows an adaptive parametric loudspeaker system 200 proposed in US patent application no. 11/558,489 "Ultra directional speaker system and signal processing method thereof (hereinafter, Kyungmin).
• Kyungmin proposes adaptively applying pre- distortion compensation to the modulating signal x(t) (i.e. the input audible signal).
• DSBAM double sided amplitude modulation
• Kyungmin proposes the use of vestigial sideband modulation (VSB) to overcome the non-ideal filtering of one of the sidebands in single sideband (SSB) modulation.
• VSB vestigial sideband modulation
• SSB single sideband
• the adaptive parametric loudspeaker system 200 comprises 1 st and 2 nd envelop calculators 202, 204 which calculate the envelops E 1 (O and E 2 (Z " ) respectively. These envelop calculators 202, 204 are injected with signals at the baseband.
• the adaptive parametric loudspeaker system 200 also comprises a square root operator 206 which computes the "ideal" envelop ⁇ E 1 (t) predicted using the Berktay's approximation (i.e. Equation (1)). The difference between ⁇ E 1 (Z”) and E 2 (t) is then used to train the pre-distortion adaptive filter 208 using the least mean square (LMS) scheme.
• LMS least mean square
• Equation (2) The coefficients a m of the adaptive filter 208 are obtained using Equations (2) and (3) as follows wherein ⁇ is an adaptive coefficient.
• aJ' t) -2(4Ejt) - E 2 (t))x(t -m) (2)
• a m (t + l) a m (t) + ⁇ a m ' (t) (3)
• the output x'(t) of the adaptive filter 208 is shown in Equation (4) as follows.
• a pre-processing technique is also proposed in US patent no. 6,584,205 (hereinafter, Croft) to improve the performance of parametric loudspeakers.
• Fig. 3 illustrates a parametric loudspeaker system 300 proposed in Croft.
• Croft proposed the use of SSB modulation as it offers the same ideal linearity as characterized by square rooting a pre-processed DSBAM modulated signal.
• Croft further proposed compensating for the distortion inherent in SSB signals using a multi-order distortion compensator.
• the multi- order distortion compensator comprises a cascade of distortion compensators (Distortion compensator 0... N-1 as shown in Fig. 3) whereby a pre-distorted signal (for example, X 1 (t) ) from one distortion compensator is used as the input to the next distortion compensator in the cascade and so on, until the desired order is reached.
• Each distortion compensator of Croft comprises a SSB modulator 302 which employs a conventional SSB modulation technique.
• the non-linear models 304 shown in Fig. 3 are based on Berktay's approximation (i.e. Equation (1)) and the system 300 proposed in Croft is based on a feed forward structure found in the multi- order distortion compensator.
• a directional sound system comprising: a plurality of equalization stages configured to equalize an input signal; and a transducer stage configured to transmit the equalized input signal; wherein the plurality of equalization stages comprises a first equalization stage configured to employ an approximated model of the transducer stage and a second equalization stage configured to compensate for differences between the approximated model of the transducer stage and an actual model of the transducer stage.
• a method for processing an input signal to a directional sound system comprising: repeatedly equalizing the input signal; and transmitting the equalized input signal; wherein a first equalization of the input signal is performed using an approximated model of the transmission and a second equalization of the input signal comprises compensating for the differences between the approximated model of the transmission and an actual model of the transmission.
• the directional sound system further comprises a modulation stage configured to modulate the equalized input signal from the first equalization stage prior to the second equalization stage, wherein the modulation stage employs a modulation technique which uses a pre-distortion term with a variable order.
• the method preferably further comprises modulating the equalized input signal from the first equalization prior to the second equalization by employing a modulation technique which uses a pre-distortion term with a variable order.
• the modulation technique which uses a pre-distortion term with a variable order.
• the addition of the pre-distortion term may reduce distortion in the demodulated signal (i.e. the audio signal output of the directional sound system).
• the amount of reduction in the distortion is dependent on the order of the pre-distortion term.
• a higher order will achieve a greater amount of reduction in the distortion.
• a higher order pre-distortion term requires an ultrasonic transducer with a higher bandwidth.
• the flexibility of the modulation technique is increased and the order of the pre-distortion term may be varied to suit the requirements of the ultrasonic transducer used in the directional sound system. For example, a lower order may be used for ultrasonic transducers with lower bandwidth whereas the order may be scaled up for ultrasonic transducers with higher bandwidth to further reduce the distortion in the audio signal output of the directional sound system.
• a sub-band approach is employed whereby the input signal is split into a plurality of frequency regions and each frequency region of the input signal is processed independently through at least one stage of the directional sound system.
• the linear nature of the frequency and phase response of the transducer stage within each sub-band may be exploited during equalization.
• equalization since equalization may be applied to each frequency region independently, the amplitude of the equalized signal in each frequency region will generally not be as low as the amplitude of the equalized signal in the full-band approach and thus, a lower amplification is required for the equalized signal in each frequency region.
• the input signal may be downsampled, thus lowering and varying the speed requirement for processing each frequency region and in turn lowering the speed requirement for processing the entire signal.
• This mixed-rate processing technique thus removes the need for high-end processors and instead, a low cost- digital signal processor can be used to implement the directional sound system.
• the modulation technique for each frequency region may be adjusted independently.
• different components with different requirements for example, different modulation techniques or different ultrasonic transducers
• the modulation technique for each frequency region may be adjusted to match the requirements of the components used for that frequency region.
• the modulation stage comprises a first modulation stage and the directional sound system further comprises: a second modulation stage configured to further modulate the modulated equalized input signal wherein a carrier frequency of the further modulated equalized input signal is dependent on a first carrier frequency in the first modulation stage and a second carrier frequency in the second modulation stage.
• a second modulation stage configured to further modulate the modulated equalized input signal wherein a carrier frequency of the further modulated equalized input signal is dependent on a first carrier frequency in the first modulation stage and a second carrier frequency in the second modulation stage.
• Having more than one modulation stage is advantageous as it divides the complexity of the system into more parts and hence, the system can be realized with relatively cheaper hardware such as analog modulators.
• using more modulation stages offers more flexibility in the selection of the carrier frequency for the modulation of the input signal since the overall carrier frequency may be adjusted by independently adjusting the carrier frequency of each modulation stage.
• Fig. 1 illustrates a parametric loudspeaker according to a first prior art
• Fig. 2 illustrates an adaptive parametric loudspeaker system according to a second prior art
• Fig. 3 illustrates a parametric loudspeaker system according to a third prior art
• Fig. 4 illustrates a parametric loudspeaker system according to an embodiment of the present invention
• Fig. 5 illustrates a modulation technique employed in a modulation stage of the parametric loudspeaker system of Fig. 4;
• Fig. 6 illustrates a parametric loudspeaker system which is a first variation of the parametric loudspeaker system of Fig. 4;
• Fig. 7 illustrates a parametric loudspeaker system which is a second variation of the parametric loudspeaker system of Fig. 4;
• Fig. 8 illustrates the Total Harmonic Distortion performance of the modulation technique of Fig. 5.
• Fig. 4 illustrates a directional sound system in the form of a parametric loudspeaker system 400 according to an embodiment of the present invention.
• the input signal 414 of the parametric loudspeaker system 400 is usually an audible sound signal.
• the parametric loudspeaker system 400 comprises a filter bank 402, a first equalization stage 404, a modulation stage 408, and a transducer stage 412 comprising an ultrasonic transducer.
• the filter bank 402 serves to split the input signal 414 into different frequency regions.
• the first equalization stage 404 serves to equalize the input signal 414 whereas the modulation stage 408 serves to modulate the equalized input signal.
• the transducer stage 412 then serves to transmit the modulated equalized input signal.
• the parametric loudspeaker system 400 also comprises a second equalization stage 406 for further equalization and an amplification stage 410 comprising an ultrasonic amplifier for amplifying the modulated equalized input signal prior to the transducer stage 412.
• both the frequency and phase response of the transducer stage 412 are compensated in the equalization stages 404, 406 of the parametric loudspeaker system 400.
• the parametric loudspeaker system 400 employs a sub-band approach whereby the input signal is split into a plurality of frequency regions (in other words, a plurality of bands) by the filter bank 402 and each frequency region of the input signal is independently processed through the first equalization stage 404, and the modulation stage 408.
• the parametric loudspeaker system 400 may be referred as a "multi- band audio beaming" system.
• the filter bank 402 serves to split the input signal 414 into different frequency regions. As shown in Fig. 4, the filter bank 402 employs N filters (Filter h ⁇ ,J ⁇ ,---h N _ ⁇ ). Each filter h t may have a different bandwidth and only frequencies lying within the bandwidth of the filter h t are allowed through the filter h t .
• the first equalization stage 404 serves to compensate for one or more expected changes in the input signal after demodulation.
• the first equalization stage 404 serves to compensate for an expected 12dB/octave slope change in the input signal after demodulation as predicted by the Berktay's approximation in Equation
• the first equalization stage 404 further serves to compensate for the frequency and phase response of the transducer stage 412 which is usually highly non-linear. As shown in
• the first equalization stage 404 comprises a plurality of equalizers (Loudspeaker
• the first equalization stage 404 employs an approximated model of the transducer stage 412 whereby . the responses of the plurality of equalizers (Loudspeaker Equalizers e ⁇ e ⁇ — e ⁇ ) are set based on an inverse of the approximated model. This approximated model may be obtained based on the product specifications of the ultrasonic transducer used in the transducer stage 412.
• the modulation stage 408 employs a modulation technique which uses a pre-distortion term with a variable order as shown in Fig. 5.
• g(t) (1 + mg(t)) sin ⁇ o t + ⁇ Z ⁇ . ' , , m 2i g 2i (t) cos ⁇ 0 t
• the modulation technique works by modulating the input g(0 with a first carrier signal sin O 0 Ho produce a main signal (1 + mg(t)) sin ⁇ o t , multiplying a pre-distortion term ]T -— 2 . m 2l g 2 '(t) with a second carrier signal
• the output g(t) comprises an additional orthogonal term
• the envelope of the modulation technique output g(t) is ⁇ ] f 2 (t) + f 2 (t) .
• Equation (1 ) the demodulated signal (or audible difference frequency) pressure p 2 (t) along the axis of propagation is proportional to the second time-derivative of the square of the envelope of the modulated signal.
• Equation (7) is obtained as follows.
• the modulation stage 408 comprises a plurality of band n modulators with each band n modulator employing the modulation technique of Fig. 5. It is possible to adjust the order q of the pre-distortion term in each band n modulator independently.
• the order q of the pre-distortion term for each band n modulator is selected according to a bandwidth of the filter h ⁇ in the filter bank 402 operably connected to the modulator. In this example, the order q is selected to be higher if the bandwidth of the respective filter of the filter bank 402 is larger.
• the frequency spectrum of the ultrasonic transducer in the transducer stage 412 is generally non-symmetrical about its resonance frequency.
• the second equalization stage 406 serves to compensate for this.
• the second equalization stage 406 further serves to compensate for the differences between an actual model of the transducer stage 412 and the approximated model of the transducer stage 412 used in the first equalization stage 404.
• the actual model of the transducer stage 412 may be obtained through experimentation.
• the second equalization stage 406 employs an adaptive filter (Loudspeaker Equalizer e w ).
• This adaptive filter is trained with a LMS algorithm using the difference between a first signal and a second signal.
• the first signal is obtained by double integrating the input signal 414 and processing this double integrated signal through an inverse of the actual model of the transducer stage 412 (i.e. "inverse transducer model") whereas the second signal is obtained using the equalized signal from the first equalization stage 404.
• the double- integrated signal and the equalized signal are modulated by sin ⁇ 0 t to match the resonance frequency of the ultrasonic transducer in the transducer stage 412. This resonance frequency is typically in the ultrasonic range.
• 406 can achieve equalization of the non-symmetrical response of the ultrasonic transducer and at the same time, compensate for the differences between the actual model of the transducer stage 412 and the approximated model of the transducer stage 412.
• Fig. 6 illustrates a parametric loudspeaker system 600 which is a first variation of the parametric loudspeaker system 400.
• the filter bank 402', the first equalization stage 404' and the modulation stage 408' of the parametric loudspeaker system 600 are identical to that of the parametric loudspeaker system 400 and thus, these parts have the same reference numerals with the addition of prime.
• the second equalization stage 602 of the parametric loudspeaker system 600 comprises a plurality of adaptive filters ( N Loudspeaker Equalizers e 0 ' , e[,• ⁇ •e N ' _ l ) with each adaptive filter trained using a corresponding inverse transducer model (Group i).
• the amplification stage 604 of the parametric loudspeaker system 600 comprises a plurality of ultrasonic amplifiers (Ultrasonic Amplifier 0, 1 , ..., N -I ) whereas the transducer stage 606 of the parametric loudspeaker system 600 comprises a plurality of ultrasonic transducers (Ultrasonic Transducer Group 0, 1 , ..., N -I ).
• Each band n modulator in the modulation stage 408' is operably connected to a filter h n in the filter bank 402' and an ultrasonic transducer (Ultrasonic Transducer Group n ) in the transducer stage 606.
• the frequency spectrum of each ultrasonic transducer in the transducer stage 606 may be non-symmetrical about its resonance frequency and the second equalization stage 602 serves to compensate for this by tuning each of the loudspeaker equalizers ( N Loudspeaker Equalizers e 0 ' , e[, -- - e N ' _ l ) to match the respective ultrasonic transducer.
• the parametric loudspeaker system 600 extends the sub-band approach to the second equalization stage 602, the amplification stage 604 and the transducer stage 606.
• the input signal 608 is first split into different frequency regions using the filter bank 402' and each frequency region of the input signal 608 is independently processed through the two equalization stages 404', 602, the modulation stage 408', the amplification stage 604 and the transducer stage 606.
• Such a structure allows the usage of different types of emitters in different ultrasonic transducers for different frequency regions.
• each band n modulator of the modulation stage 408' may be independently adjusted to match the requirements of the respective ultrasonic transducer (Ultrasonic Transducer Group n ) operably connected to it.
• the output of each band n modulator may also be independently adjusted according to the bandwidth of the filter h n in the filter bank 402' operably connected to it.
• each band n modulator may employ the modulation technique as shown in Fig. 5 and the order q of the pre-distortion term for each band n modulator may be selected based on a bandwidth of the corresponding filter h n in the filter bank 402' and a bandwidth of the corresponding ultrasonic transducer (Ultrasonic Transducer Group n ).
• each band n modulator in the modulation stage 408' may use a different carrier frequency (i.e. a different ⁇ 0 as shown in Fig. 5) such that the carrier frequency for each modulator matches the resonance frequency of the respective ultrasonic transducer (Ultrasonic Transducer Group n ).
• a different carrier frequency i.e. a different ⁇ 0 as shown in Fig. 5
• Fig. 7 illustrates a parametric loudspeaker system 700 which is a second variation of the parametric loudspeaker system 400.
• the parametric loudspeaker system 700 is similar to the parametric loudspeaker system 600 and thus, the same parts will have the same reference numerals with the addition of prime.
• the parametric loudspeaker system 700 not only comprises a first modulation stage 408", it further comprises a second modulation stage 702 for further modulating the modulated signal.
• each frequency region of the input signal is independently processed through the first and second modulation stages 408", 702.
• the carrier frequency of the further modulated signal is dependent on a first carrier frequency in the first modulation stage 408" and a second carrier frequency in the second modulation stage 702.
• the second modulation stage 702 also comprises a plurality of modulators (Band N Modulator, Band N + 1 Modulator... Band 2 N - 1 Modulator) and the output of each modulator may be independently adjusted to match the requirements of the ultrasonic transducer operably connected to it.
• the output of each modulator may also be independently adjusted according to the bandwidth of the filter h n of the filter bank 402" operably connected to it. Having more than one modulation stage is advantageous.
• the carrier frequency in each modulation stage can be lowered without lowering the overall carrier frequency.
• the sampling frequency of the input signal may be lowered and the computational requirement for processing the input signal may be reduced.
• having two modulation stages 408", 702 divides the complexity of the system 700 into two parts and hence, the system 700 can be realized with relatively cheaper hardware such as analog modulators. For example, users may implement the first modulation stage 408" on a readily available computer despite its low sampling frequency since the overall carrier frequency may be increased using the second modulation stage 702 which may be implemented on, for example, inexpensive analog modulators external to the computer.
• the additional modulation stage 702 offers more flexibility in the selection of the carrier frequency for the modulation of the input signal since the overall carrier frequency may be adjusted by adjusting either the first carrier frequency in the first modulation stage 408" or the second carrier frequency in the second modulation stage 702, or both of these carrier frequencies. This is particularly useful in the sub- band approach especially when the ultrasonic transducers used for different frequency regions have different resonance frequencies.
• the parametric loudspeaker system comprises a single modulator in the modulation stage and a single ultrasonic transducer in the transducer stage.
• the single modulator may employ the modulation technique as shown in Fig. 5 with the order q selected based on the frequency range (i.e. bandwidth) of the input signal and the bandwidth of the single ultrasonic transducer.
• the modulator can still be adapted to different types of ultrasonic transducers with different frequency responses. This helps in reproducing directional sound with minimum distortion.
• the sub-band approach is still preferable since by using several band n modulators employing the modulation technique in Fig. 5, the order q for each band n modulator can be adjusted separately and thus, a higher reduction of distortion can be achieved.
• the parametric loudspeaker system may comprise a plurality of adaptive filters in the second equalization stage and only a single ultrasonic transducer in the transducer stage.
• the parametric loudspeaker system may comprise a single adaptive filter in the second equalization stage and a plurality of ultrasonic transducers in the transducer stage.
• this example is not preferable.
• the filter bank and first equalization stage of the parametric loudspeaker system 400, 600, 700 may be combined into a single equalization stage whereby the single equalization stage serves to split the input signal into different frequency regions, compensate for one or more expected changes in the input signal after demodulation and at the same time, compensate for the frequency and phase response of the transducer stage.
• the parametric system may comprise more than two equalization stages and may also comprise more than two modulation stages. Each of these modulation stages may or may not employ the modulation technique of Fig. 5. In one example, only some modulators in any one modulation stage employ the modulation technique of Fig. 5.
• Preprocessing methods to reduce the distortion in parametric loudspeakers have previously been suggested.
• these preprocessing methods are based on a single-band approach, whereby a single pre-processing method and modulation technique is applied to the entire frequency range of the signal.
• ultrasonic emitters used with these preprocessing methods.
• the embodiments of the present invention can address both the single-band problem and the problem arising due to the difference in the frequency responses of different ultrasonic emitters.
• An adaptive approach is also incorporated in the embodiments to compensate for the deficiency in the ultrasonic emitters.
• the modulation stage employs a modulation technique known as Modified Amplitude Modulation g (MAMg) which uses a pre-distortion term with a variable order.
• MAMg Modified Amplitude Modulation g
• an orthogonal term formed by multiplying a pre-distortion term with an orthogonal carrier signal
• This is different from the typical amplitude-based modulation techniques used in the prior art.
• the THD values achieved by the MAMg scheme reduces rapidly with the rate of reduction in THD values decreasing as the relative bandwidth increases beyond 10%. This indicates that the MAMg scheme works well even for ultrasonic emitters with low relative bandwidth. Furthermore, not only does the MAMg scheme achieve low THD values, it also has the flexibility of scaling up its order q to further reduce the THD values for wider bandwidth ultrasonic emitters.
• the addition of the pre-distortion term can greatly reduce distortion in the demodulated signal (i.e. the audio signal output of the parametric loudspeaker system).
• the amount of reduction in the distortion is dependent on the order of the pre-distortion term.
• a higher order will achieve a greater amount of reduction in the distortion.
• a higher order pre-distortion term requires an ultrasonic transducer with a higher bandwidth.
• the flexibility of the modulation technique is increased and the order of the pre- distortion term may be varied to suit the requirements of the ultrasonic transducer used in the parametric loudspeaker system. For example, a lower order may be used for ultrasonic transducers with lower bandwidth whereas the order may be scaled up for ultrasonic transducers with higher bandwidth to further reduce the distortion in the audio signal output of the parametric loudspeaker system.
• the embodiments of the present invention use a sub-band approach. Unlike the conventional fullband approach, the embodiments of the present invention are able to solve the problems in a more detailed manner by partitioning the input signal into smaller frequency regions (or bands) and using a "divide-and-conquer" approach to reduce the distortion found in these smaller regions. As such, different algorithms may be used to remove the distortion found in different frequency regions, thereby enhancing the quality of the audio sound produced by the parametric loudspeaker system.
• the embodiments of the present invention can exploit the linear nature of the frequency and phase response of the transducer stage within each subband.
• the compensation for the frequency and phase response of the transducer stage is simplified using the embodiments of the present invention.
• the amplitude of the input signal may be higher in certain frequency regions whereas it may be lower in other frequency regions.
• equalization is achieved by lowering the amplitude of the input signal in high amplitude frequency regions to match the amplitude of the input signal in the lowest amplitude frequency region (i.e. the frequency region in which the amplitude of the input signal is the lowest).
• the signal is amplified after equalization. This is highly undesirable due to the low efficiency in electrical to acoustic conversion in ultrasonic transducers.
• the sub-band approach in the embodiments of the present invention avoids these issues arising in typical full-band equalization. Using the sub-band approach, the equalization is applied to each frequency region independently and therefore the amplitude of the equalized signal in each frequency region will generally not be as low as the amplitude of the equalized signal in the full- band approach. Thus, a lower amplification is required for the equalized signal in each frequency region.
• the embodiments of the present invention using the sub-band approach provide a significant reduction in harmonic distortion and in the intermodulation distortion as compared to the traditional full-band approach.
• the input signal may be downsampled, thus lowering and varying the speed requirement for processing each frequency region and in turn lowering the speed requirement for processing the entire signal.
• This mixed- rate processing technique thus removes the need for high-end processors and instead, a low cost digital signal processor can be used to implement the multi-band Audio Beaming System in the embodiments of the present invention.
• ultrasonic transducers with higher bandwidth are more desirable, they are usually more expensive.
• the sub-band approach allows the use of different types of ultrasonic transducers in the same system, thus allowing the use of cheaper ultrasonic transducers with lower bandwidth for input frequencies which are less important. This in turn lowers the cost of the system.
• the embodiments of the present invention can also be scaled, for example by the manufacturer, to fit the required applications.
• the pricing of the system may also vary according to the scaling. Thus, the products can be differentiated.
• two or more modulation stages may be provided. This allows the embodiments to be realized with relatively cheaper hardware such as analog modulators. This also offers more flexibility in the selection of the carrier frequency for the modulation of the input signal since the overall carrier frequency may be adjusted by independently adjusting the carrier frequency in each modulation stage.
• two or more equalization stages may be provided in the embodiments of the present invention. This is advantageous as the initial equalization stages can implement a coarser equalization of the input signal whereas the later equalization stages can implement a finer equalization of the input signal. In this way, equalization of the input signal is performed in a more efficient and accurate manner.
• the embodiments of the present invention thus provide a comprehensive approach in reducing distortion in a parametric array.
• the embodiments of the present invention provide the ability to step up or step down the order of the modulation technique to suit different types or different bandwidths of ultrasonic emitters (or transducers). Furthermore, by using the subband approach, the order of the modulation technique can be tuned based on the need in each frequency region.
• the embodiments of the present invention provide the ability to apply a more relevant or direct pre-processing scheme for each frequency region of the input signal, thus providing a multi-band distortion reduction approach for audio beaming.
• the embodiments of the present invention provide the capability to match the bandwidth of the ultrasonic transducer to achieve low distortion.
• the embodiments of the present invention provide sub-band based equalization of the frequency and phase response of the transducer stage. In contrast to the full-band approach, this avoids the problem of severely reducing the output sound level of parametric loudspeakers when implementing equalization
• the embodiments of the present invention achieve efficient computational complexity
• the embodiments of the present invention achieve flexibility and may be implemented at a lower cost through the use of multiple modulation stages

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• Engineering & Computer Science (AREA)
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• Acoustics & Sound (AREA)
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• Multimedia (AREA)
• Audiology, Speech & Language Pathology (AREA)
• General Health & Medical Sciences (AREA)
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• Obtaining Desirable Characteristics In Audible-Bandwidth Transducers (AREA)

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• 2010
  • 2010-08-25 KR KR1020127007485A patent/KR20120054082A/ko not_active Application Discontinuation
  • 2010-08-25 JP JP2012526692A patent/JP5450820B2/ja not_active Expired - Fee Related
  • 2010-08-25 US US13/391,000 patent/US8891783B2/en not_active Expired - Fee Related
  • 2010-08-25 SG SG2012007571A patent/SG178241A1/en unknown
  • 2010-08-25 WO PCT/SG2010/000312 patent/WO2011025461A1/en active Application Filing
  • 2010-08-25 EP EP20100812417 patent/EP2471277A4/de not_active Withdrawn

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